I've actualy been meaning to write a blogpost about it. Some of the things I've noticed from being on and off cigarettes:
Cigarettes (edit: nicotine, actually) are a depressant (EDIT: and a stimulant), they make the world feel less colourful (LITERALLY), less "noisy".
EDIT: I realize I'm being psuedosciency and imprecise with the previous statement. I'm not sure about the precise distinctions between stimulant/upper/downer, etc. What I CAN tell you is that cigarettes have the following effects for me and my smoker peers, as far as I've known ->
When I'm on cigarettes, my field of vision narrows, colours become duller, I'm marginally more focused on specific tasks, better able to "tune out 'random noise'". Whenever I go off cigarettes, the world feels a little overwhelming and chaotic for a while- my skin is more sensitive to the wind, my eyes are more sensitive to the light, the whole nervous system just gets overwhelmed (because the cigarettes kinda suppess them, in my opinion.)
The short-term gain in focus is nothing great. You can get it by sitting in silence for a minute, breathing deeply, psyching yourself up with a youtube video. Not worth the damage you do to your eyes, nose, gums, teeth, not worth the cough, not worth the smell, the taste... all of that is underreported because there's a "frog in boiling water" effect- the damage is so cumulative that it feels tolerable.
Yes, I remember when cigarettes helped focus my thoughts as a writer, or over conversations with friends... but honestly, taking a walk achieves superior returns, on both counts.
--
Commander Shepard: "What's your opinion of this? The drugs, I mean."
Legion: "These substances enhance short-term performance at the expense of long-term platform survivability. It is fundamentally similar to "over-clocking" geth hardware. We do so whenever necessary. However, should our platform be damaged by overclocking, we can be repaired. Why an organic would choose this is puzzling."